Games : Why did Valve make it so you can't trade Steam trading cards for two weeks after you purchase them?

Okay, before this is marked as being subjective (as the ominous tip box in the corner of my screen seems to be suggesting) I would like to say the I'm looking for a specific Valve policy here-something that's quotable. And a little bit more in depth that the "we're trying to prevent fraud" notice on the Steam Marketplace.